Fix typos in 1.8 release notes.

  parameters. Internally, Django will continue to provide the

  ``pk`` parameter in ``params`` for backwards compatibility.

* ``UserCreationForm.errors_messages['duplicate_username']`` is no longer used.

* ``UserCreationForm.error_messages['duplicate_username']`` is no longer used.

  If you wish to customize that error message, :ref:`override it on the form

  <modelforms-overriding-default-fields>` using the ``'unique'`` key in

  ``Meta.errors_messages['username']`` or, if you have a custom form field for

  ``Meta.error_messages['username']`` or, if you have a custom form field for

  ``'username'``, using the the ``'unique'`` key in its

  :attr:`~django.forms.Field.error_messages` argument.